The present invention relates to the removal of contaminants, particularly suspended and organic materials, from waste water, and is of particular but by no means exclusive application in the purification of sewage.
Existing processes for the treatment of waste water, for the removal of suspended material and other contaminants, include mechanical, biological and physico-chemical processes, or combinations of these. Mechanical processes include filtration techniques, or the removal of solid suspensions through settling and differences in specific gravities. Biological techniques employ microorganisms to remove contaminants, particularly dissolved organic substances, from the liquid waste by incorporation into a biomass which is more easily separated from the liquid stream than the original contaminants. A significant portion of this biomass is generally converted to inorganic materials either before or after separation from the liquid stream. Physico-chemical techniques exploit the reactivity of certain minerals or other chemicals with organic or inorganic contaminants and may be used, for example, to supplement a biological purification technique, such as by chemical addition for improved precipitation and/or flocculation (e.g. the addition of lime for pH adjustment).
One existing physico-chemical technique is described in EP 177543, which discloses a process for the removal of suspended solids, biogenic nutrients and dissolved metal compounds from water contaminated with organic and/or inorganic substances, by dosing a completely mixed type activated sludge reactor with an agent of grain size less than 200 xcexcm and other characteristics specified in that document, and which contains a minimum of 50 mass percent of rock granules containing at least 25 mass percent finely ground natural zeolite containing preferably clinoptilolite and/or mordenite. In a preferred form of the technique disclosed in this European Patent, sewage is passed through a primary settling tank, followed by a mixing tank and then an absorption zone, and an aeration basin, and finally through a secondary settling tank. In the settling tank, primary effluent and activated sludge are separated and a fraction of the settled sludge is recirculated into the mixing tank.
Zeolite of specified grain size is fed into the mixing tank so that the water leaving the secondary settling tank has a considerably decreased level of suspended material. Pre-treated sewage leaving the secondary settling tank is led through one of several zeolite beds that are filled with suitably prepared material of proper grain size and of high clinoptilolite and/or mordenite content.
These beds are flushed with purified water to remove sludge floccules from the upper layers; the flush-water is then fed back to the primary settling tank.
However, existing processes (employing various suspended growth media, including ground zeolite) have a number of disadvantages including that the growth media are separated from waste sludge prior to disposal, may usually be of synthetic manufacture, and do not have optimal surface area and pore volume characteristics. The process of EP 177543 does not separate the growth media from the waste sludge prior to disposal, it proceeds only up to a complete biological oxidation of contaminating carbon compounds.
Processes of this kind depend on the settleability of suspended material in the waste water. A particle of matter in suspension in a liquid will tend to settle under quiescent conditions if the specific gravity of the particle is greater than that of water. Passage of the particle through the liquid will be resisted by frictional forces, and hence the settling rate will be appreciable only if there is a reasonable difference in specific gravity between the particle and the liquid.
Activated sludge is a flocculant suspension consisting predominantly of bacteria. Because bacterial density is very close to that of water individual bacteria will not settle, and separation of activated sludge from water is dependant on the formation of aggregates containing many bacteria (i.e. flocs).
The way in which flocs settle depends on both their nature/quality and their concentration in the liquid. Many settleability parameters have been proposed in an effort to measure sludge quality as a specific entity unrelated to concentration, but with limited success.
In the course of such physico-chemical processes, the settleability of the sludge may be used to determine the sludge quality and hence optimize treatment of the sludge. Numerous parameters are used for assessment of the settleability of activated sludges, but unfortunately most of these fail to define sludge quality unambiguously, and even when supplemented by additional information regarding the test conditions the results are often not very helpful.
Part of the problem is that available parameters are often applied for purposes other than those for which they were intended, while in some cases standardisation of procedures leads to improved consistency in measurement at the expense of applicability to operating situations.
As with quality parameters in general, different parameters are required for different aspects of settleability. The permissible rise rate in clarifiers, for example, relates directly to the settling rate of the sludge while satisfactory decant of intermittently aerated plants depends on the distance the sludge has settled before decant commenced (and not necessarily on whether it did this at a uniform rate or not).
The most commonly used parameters for assessment of activated sludge settleability are:
the sludge volume index (SVI)
the stirred sludge volume index (SSVI)
the mass concentration of suspended solids (MLSS)
parameters known as V0 and n (or k), which are used to determine the xe2x80x9csteady ratexe2x80x9d settling velocity of the sludge at various concentrations.
Both SVI and SSVI suffer from difficulties in that they are affected by sludge concentration to an extent which is not completely predictable (and hence they do not uniquely identify the quality of the sludge).
V0 and n do seem to reflect the quality of a given sample of sludge, but the test procedure is laborious and may not result in unambiguous values, particularly if not carried out over a suitable range of sludge concentrations. There is also little information available on the changes in V0 and n response to changes in plant conditions. As such, there are some doubts about application of the results to operating situations.
A number of researchers have attempted correlations between V0 and n and either SVI or SSVI, andxe2x80x94while general correlations do seem possiblexe2x80x94true correlation should probably not be expected because the parameters do not really measure the same thing (reasonable correlation being a reflection of influence by similar factors rather than a true relationship).
The shortcomings with SVI were addressed by Stobbe (1964), who recognised that the SVI is essentially independent of concentration at low sludge concentrations and developed the Diluted Sludge Volume Index (DSVI). However, the DSVI may, in practice, be dependant on the concentration of the sludge under examination, and so may not produce a unique value.
It is also recognized that different types of settling occur, and four distinct settling xe2x80x98zonesxe2x80x99 have been designated on the basis of floc behaviour. These are known as: 1) the free settling zone, 2) the hindered settling zone, 3) the compression zone, and 4) the transition zone. Not all settleability parameters are appropriate in all zones.
Thus, care must be taken in employing settleability parameters that can correctly represent the characteristics of the particular settling zone being dealt with. In addition, the applicable regime of any particular settleability parameter depends on a number of factors. For example, when using SVI it is necessary to determine the MLSS of the sample (which is usually not possible on site); at low to moderate MLSS, the SVI increases with MLSS, but with a proportionality that is difficult to predict for any given MLSS; and there is a maximum SVI which cannot be exceeded and this maximum decreases as the MLSS increases. As a consequence of this changing maximum SVI, samples that settle poorly appear to approve in SVI as MLSS increases, but in reality remain poor settling sludges.
One existing method for overcoming this last limitation of the SVI (due to the effects of the MLSS concentration) determines the SVI at a specified MLSS, such as 3,500 mg/L. This is usually done by testing a variety of dilutions, and extrapolating to the required concentration, but thereby increases the amount of testing required. This procedure is satisfactory provided that the sludge settles reasonably well around the specified MLSS, but is of little value is settleability is low at the specified MLSS.
It should also be noted that it is at all times desirable to minimise the amount of testing required to determine settleability, as running such tests are both time consuming and expensive owing to the cost of the experimental equipment. For example, a cylinder and stirrer for conducting a settleability measurement may cost around AU$1,200.
Thus, it is an object of the present invention to provide an improved process for removing dissolved and suspended material waste water. It is further object of the present invention to provide an improved process for optimizing such a process by means of an improved sludge settleability parameter.
The present invention provides, therefore, a process for treating waste water in a reactor in order to remove contaminants, including:
mixing said waste water with a biomass, said biomass for consuming a quantity of said contaminants or adsorbing said contaminants;
dosing said waste water with a micronized zeolitic material;
mixing said zeolitic material with said biomass;
allowing solids to settle from said waste water;
discharging resultant effluent; and
maintaining said zeolitic material at a sufficient level relative to said biomass to allow colonization of said zeolitic material by micro-organisms;
wherein a significant portion of said micronized zeolite material has a grain size less than or equal to 100xcexcm and said zeolitic material acts as a suspended growth support carrier for said micro-organisms.
Preferably said zeolitic material is natural zeolite.
Thus, the process of the present invention employs micronised zeolite (i.e. of particles size less than 100 xcexcm) to act as a suspended growth-support carrier for micro-organisms in the waste water. As presently understood, the zeolitic material promotes aggregation of sewage matter (and especially micro-organisms) suspended in the waste water. The micro-organisms colonise the surface and pores of the zeolitic material and metabolise organic and nitrogenous substances which are attracted to the zeolite particles by adsorption or cationic exchange, or other mechanisms. It has been surprisingly found that micronised zeolite provides unexpected advantages over the use of zeolite in a merely ground form: micronised zeolite appears to provide an optimal surface area for such adsorption and other mechanisms to take place, and a significant fraction of the micronised material is retained in a suspended state. Micronisation at this level also appears to lead to a homogenous suspension in water for both mixing and dosing purposes. Further, the process of the present invention continues beyond oxidation of carbon to oxidation of nitrogen and to denitrification, whereasxe2x80x94as discussed abovexe2x80x94the process EP 177543, for example, proceeds only up to a complete biological oxidation of contaminating carbon compounds.
The biomass preferably comprises microscopic organisms.
The reactor may consist of one or more compartments, and the process may include recycling the biomass from one compartment to another.
A portion of the biomass may be aerobic, anoxic or anaerobic, or a mixture of these, or may be one of these during part of the process but not during other parts of the process.
The micronised material may be added as a dry powder or as a slurry. The micronised material may be added at a uniform rate, or adjusted to match waste water inflow rate, or added in slug doses once or a number of times.
The process may include dosing directly into a reactor in which the process is performed, or into a sludge recirculation line, or into a channel, pipe or container upstream of the reactor.
The process may include separating said solids in the reactor during a period of quiescence or in a separator unit, such as a clarifier, into which the contents of the reactor have previously been transferred for separation.
The process preferably includes monitoring the settleability of the solids.
The process may include retaining the biomass in the reactor for from about one to two days up to about thirty to forty days or longer, and regularly wasting a proportion of the biomass to compensate for the introduction of new inputs and growth of the biomass. While some immediate benefits may be obtained almost immediately after dosing with the micronised material, it has been surprisingly found that further benefits (which depend on the structure of the biomass) are obtained but do not become apparent until some weeks after dosing commences.
The present invention also provides a method for optimizing the separation of solids from waste water in a waste water treatment process, including:
diluting a plurality of samples of said waste water to different dilutions to form a plurality of diluted samples;
determining the unstirred sludge volume index for each diluted sample;
extrapolating these determinations of unstirred sludge volume index to substantially zero sludge concentration to obtain a zero concentration index;
multiplying the zero concentration index by the concentration of solids in the waste water to obtain a unitless parameter indicative of sludge quality;
controlling the process to minimise this parameter.
This parameter is referred to hereinbelow as the ZF Index. The ZF Index is calculated as follows:
The Base SVI is determined by:
diluting the original sample so as to obtain several subsamples of known concentration
determining the unstirred sludge volume index for each of these diluted samples
extrapolating these determinations of sludge volume index to substantially zero concentration The SVI at this substantially zero concentration is the Base SVI.
Preferably the ZF Index is held at a value of 0.50 or less, and more preferably at a value of 0.3 or less.
Preferably dilution is with effluent from said process.
Preferably the sludge volume index of each diluted sample is determined using the 30 minute settled level.
In some applications the method may include introducing a time delay in testing the settleability, such as necessitated by testing the settleability away from the reactor or off-site, or by using tap water for dilution rather than effluent. Preferably volumes of one liter are used in the settleability tests, but this may be varied according to available time and equipment and similarly, time or equipment constraints may mean that fewer than five samples are used, but preferably at least five are used.
Good settleability (a very low risk of solids loss into the effluent) will be obtained if the ZF Index is less than about 0.3, while reasonable settleability (a low to moderate risk of solids loss into the effluent) is obtained if the ZF Index is between about 0.3 and about 0.50. As the ZF Index increases the settleability of the sludge decreases, and once this value exceeds about 0.50 there is a very real risk of solids loss into the effluent.
These relationships hold whether the process being monitored is that described above or not; the advantage of the above process is that, under most circumstances, it causes an improvement in the ZF Index.
Once the ZF Index and Base SVI [or zero concentration index] have been determined, an operating MLSS can be calculated which, if adopted, will allow the plant to operate with either xe2x80x9cgoodxe2x80x9d or xe2x80x9creasonablexe2x80x9d settleability (i.e. which will allow the risk of solids loss into the effluent to be estimated.)